The need for image protection has been increasing in recent years and as a result at least two forms of image metadata have been growing in importance. These forms of image metadata are firstly, authentication information, which is used to determine whether an image has been altered since acquisition, and secondly, intellectual property rights information (e.g. copyright labelling), which is used to identify the party or parties having intellectual property rights in the image. Conventional techniques for associating these forms of image metadata often have conflicting requirements, which presents several problems when both of these forms of image metadata are to be included in a single image.
As an example, watermarks can be used to embed intellectual property rights information into image data. Such watermarks need to be robust to modification of the watermarked image data through image processing techniques such as scaling, rotation, brightness, contrast adjustments and file format conversions. In contrast, watermarks used for authentication of an image are designed to be fragile to modification, as it is by the absence, or partial absence of these watermarks, that alterations to source image data can be identified. As a consequence of these different requirements, watermarking techniques used for embedding intellectual property rights and for authentication is invariably very different.
Interference between different watermarks and/or added degradation of image quality can occur when different types of watermarks are used together. Therefore, it is generally undesirable to use more than one type of watermark within a single image. However, one known watermarking technique referred to as the Secure Digital Music Initiative (SDMI) has proposed a model that uses a combination of robust and fragile watermarks in source image data. The concept behind SDMI is that the fragile watermark is destroyed in the process of copying the source image data while the robust watermark remains. The SDMI technique is however undesirable when used just for the purpose of authentication. In particular, the use of fragile watermarks can result in modifications to an image that may impact on the compressibility and/or the visual quality of the image. Also, since fragile watermarks are by their very nature fragile, these watermarks can be lost completely if the image is modified making it difficult to provide additional information to a user about the type or extent of modification.
In order to address the above mentioned problems with conventional watermarking techniques, the Joint Photographic Experts Group (JPEG) 2000 and the Moving Picture Experts Group (MPEG) 4 standards, proposed to embed copyright information as metadata in a file wrapper associated with the image data. However, copyright information embedded, as metadata in a file wrapper is potentially open to malicious modification or malicious or accidental removal during processes such as file format conversion.
In addition, copyright information embedded using watermarks is not easily visible to applications software and there is no way of guaranteeing that the watermark was not inserted fraudulently, say after stripping of copyright metadata tags. This is of particular concern in systems, which incorporate automatic billing based on embedded rights information.